Modern adhesive materials and techniques provide clinicians with conservative treatment options that preserve tooth structure, while improving longevity and aesthetics.1 Laboratory-processed inlays fabricated with porcelain or composite resin restore mechanical and biological function, while achieving optimal aesthetics with minimal tooth reduction. A conservative preparation design can be used, because the adhesive procedure strengthens the cusps and provides additional support for the prepared tooth. Additionally, laboratory-fabricated intracoronal restorations provide precise marginal integrity, ideal proximal contacts, wear resistance similar to tooth structure, reduced polymerization shrinkage, excellent anatomical morphology, and optimal aesthetics.1,2
The first part of this article discusses the consideration factors and clinical attributes for the use of direct composite systems as intracoronal restorations.3 This second part will discuss the consideration factors for the use of indirect systems. A comparison of the attributes and capabilities of porcelain versus processed composites will provide a guide for proper treatment selection by the patient, laboratory technician, and restorative dentist.
Consideration Factors for Indirect Systems
The clinical success of a bonded restoration requires one to ensure proper function, aesthetics, biocompatibility, and longevity.4 Attaining these four criteria begins at the adhesive interface. A restorative material, properly bonded to enamel and dentin, will reduce marginal contraction gaps, microleakage, marginal staining, and caries recurrence; reinforce tooth structure; and dissipate and reduce functional stresses across its interface throughout the tooth, while improving aesthetics and wear resistance.5 Several factors that can influence this adhesive joint include: thickness of the resin cement, restoration fit, stabilization of the hybrid layer, strength and durability of the interface between the resin cement and the processed restoration, and wear of the luting cement.
Consideration factors for the utilization of laboratory-processed inlays in medium to larger restorations include: polymerization shrinkage, anatomical morphology, and cavity preparation and dimension. Medium restorations are those where the preparation isthmus is one half of the distance from the central fossa to the cusp top; larger restorations are preparations that exceed this dimension and approach the cusp tip. Since laboratory-processed inlays are fabricated on a model, the only effects of polymerization shrinkage are attributed to the minimal composite resin cement shrinkage at cementation. Anatomical details and occlusion can be controlled to a higher level on the laboratory model and with the use of a microscope. In addition, the surface finish of the proximal surfaces can be polished to a greater degree and can provide an ideal contour and positive contact. Cavity preparation requires the removal of tooth structure (ie, undercuts) for a proper path of insertion and adaptation of the restoration to the cavity walls and for ease of insertion and removal during fabrication. The cavity dimension includes medium to larger preparations and provides an alternative solution consideration for cuspal coverage (ie, onlay) when the isthmus preparation exceeds one half of the distance from the central fossa to cusp tip. Since both of these restorative systems can provide predictable clinical results, a comparison of the attributes and capabilities of porcelain versus processed composites will allow for proper treatment selection by the restorative dentist and laboratory technician.
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Porcelain Versus Processed Composites
Since occlusion is equilibrated after cementation, the processed composite resin offers an advantage over porcelain because of the former’s ability to be polished intraorally. It is more difficult to establish a highly polished surface intraorally on porcelain after the glaze has been removed.6 This unpolished surface has been shown to increase the wear of the opposing dentition.7
Porcelain is not as elastic as processed composite resin and, therefore, does not tolerate elastic deformation,1 which can result in fracture of the ceramic margins at try-in. Porcelain has a high resistance to compression and a low resistance to flexion and traction and, hence, is fragile when subjected to tensile stresses.8 This presents a challenge for some inlay preparations as not all preparations provide the compression required for the ceramic material. The flexural strength of second-generation composite resin is in the range of 120 MPa to 150 MPa, which is higher than that of feldspathic ceramic (ie, 65 MPa). This slight elasticity of the composite resin helps to absorb some of the strains and thereby protects the adhesive bond at the tooth-restorative interface.1
Porcelain is harder than tooth structure and, when not polished properly, has the potential to quickly abrade teeth, whereas second-generation composite resins are softer and have a more favorable wear compatibility with the opposing natural dentition.6
Porcelain restorations have the potential for microgaps at the tooth restorative interface; second-generation composite restorations can be made with relatively small gaps. These cavosurface margins are a weak point of the ceramic restoration.6
Porcelain modifications such as those required for contacts and fractured margins are time-consuming chairside procedures, whereas indirect resin restorations can be easily modified chairside.9
Monochromatic Versus Polychromatic
Injectable ceramics are monochromatic, enabling color to be altered with external stains, which can be removed with occlusal adjustment or occlusal wear. Second-generation composite resins can be internally layered for a polychromatic effect.
Composite materials have shown a greater capacity to absorb compressive loading forces and reduce the impact forces by 57% more than porcelain. Composites, therefore, transmit less of the applied load to the underlying tooth structure.10
Composite inlays have excellent marginal integrity because of the similar thermal expansion rate as the luting cement. Conversely, the variation in coefficients of thermal expansion for porcelain inlays and the composite luting cement can result in an increased width of the luting gap.11
Although laboratory-processed composite resins provide important advantages in many situations, there are several factors that should be considered for the use of porcelain intracoronal restorations. Those factors include the efficiency and ease of fabrication in the laboratory as a result of advanced technology (eg, injectable ceramics, CAD/CAM systems), the availability of these systems in laboratories, and the technician’s expertise with this technology. Composite systems for CAD/CAM technology are not as common. In addition, porcelain systems are unsurpassed in stability of color, gloss, and wear resistance (Table 1).12
The demand for tooth-colored posterior restorations continues to increase. Combined with the declining use of traditional materials for these indications, indirect restorative systems broaden the scope of restorative modalities that are available to assist the patient, the technician, and the dentist in making informed decisions for different clinical situations.1
*Assistant Professor, Department of Restorative Dentistry and Biomaterials, University of Texas Health Science Center Dental Branch, Houston, TX; private practice, Houston, TX.
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- Meyer JM. In vitro resistance to fracture of porcelain inlays bonded to tooth. Quintessence Int 1990;21(10):823-831.
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- Gracis SE, Nicholls JI, Chalupnik JD, Yuodelis RA. Shock-absorbing behavior of five restorative materials used on implants. Int J Prosthodont 1991;4(3):282-291.
- Fuhrer N. Restoring posterior teeth with a novel indirect composite resin system. J Esthet Dent 1997;9(3):124-130.
- Craig RG, Ceramics. In: Restorative Dental Materials, 10th Ed. Mosby, St. Louis 1997; 482-483.